The present invention relates to an image forming apparatus having a member for removing electric charge on a transfer medium after transfer and to an image forming apparatus in which a transfer bias is applied by constant-current control to transfer an image from an image carrier to a transfer medium.
Conventionally, in an image forming apparatus such as a copying machine, a toner image formed through developing process is transferred to a transfer paper sheet by applying transfer electric field. When there is charge on the transfer paper sheet, toner may flow and adhere to a transfer paper sheet so as to cause image deterioration. To prevent this, the charge on the transfer paper sheet is removed by a charge removal brush (Japanese Patent Publication No. H06-77170) or by a charge removal needle placed at the position just after the transfer by a transfer roller (Japanese Patent No. 2759487 (FIG. 7)).
By the way, as a transfer paper sheet after transfer is fed through a guide member in a state that the charge removal is insufficient, the transfer paper sheet may collide with ribs of the guide member so as to produce linear image irregularities due to static electricity. For example, in a color image forming apparatus employing an intermediate transfer medium, a paper sheet pressed and fed between the secondary transfer roller and the intermediate transfer medium produces paper powder due to friction during nip. Particularly, since a transfer paper sheet which has been passed through a fixing device has been greatly damaged by pressure and heat of rollers, the transfer paper sheet produces more paper powder. Such paper powder should be deposited on the charge removal brush or the charge removal needle because of long-term use, thus significantly reducing the charge removal capability. This must be a cause of image irradiation. Conventionally, to improve the charge removal capability, a voltage of a bias polarity opposite to that of the voltage of bias polarity for transfer is applied to the charge removal member. The charge removal member therefore functions as a dust collector for paper powder floating within the apparatus so that dusts adhere to the charge removal member and are difficult to be cleaned. This must be also a cause of image irregularities. However, nothing for solving the aforementioned problems is taught and disclosed in the aforementioned prior art documents.
A method for preventing the occurrence of transfer failure caused by resistance value change of a transfer member due to environmental changes has been proposed which comprises detecting temperature and relative humidity by a temperature sensor and a humidity sensor, calculating a value related to moisture content from the detected temperature and the detected relative humidity, selecting an environmental partition according to the calculated value related to moisture content and the relative humidity, and determining a transfer output value on the basis of the environmental partition (Japanese Patent Unexamined Publication No. H06-250532).
In addition, a color image forming apparatus has been also proposed which controls the constant-current value, the highest voltage value, and/or the lowest voltage value of a transfer bias applying means on the basis of temperature and humidity detected by a temperature/humidity sensor (Japanese Patent Unexamined Publication No. H11-288184).
In both the aforementioned prior art documents, however, the transfer bias control is conducted by detecting the temperature and the humidity so that there is a problem of increasing the cost because of the provision of the humidity sensor. Since the optimal bias value depends on the humidity, good bias range for a certain temperature is not ensured if there is no humidity information. This causes the transfer failure. Particularly, in case of constant-voltage control, the variation in optimal voltage according to the humidity is so large. The humidity information is absolutely necessary.